将具有相同 id 但不同值的行合并到每个 id 的单行中

Question

I have a query that finds categories by product id. Note: Some products can have multiple categories. My current query returns the same ids for different categories

SELECT 
  oc_product.product_id, 
  oc_category_description.name 
FROM 
  oc_product 
  LEFT JOIN oc_product_to_category ON oc_product.product_id = oc_product_to_category.product_id 
  LEFT JOIN oc_category_description ON oc_product_to_category.category_id = oc_category_description.category_id;

Result:

product_id	category_name
100	storage
100	SSD
100	HDD
200	storage
200	SSD
200	HDD

I would like to merge the rows with the same product_id and add each category_name as an extra column to the left.

Desired output:

product_id	category_name	category_name 2	category_name 3
100	storage	SSD	HDD
200	storage	SSD	HDD

Answer 1

The columns of an SQL query must be fixed at the time the query is parsed. A query cannot append more columns as it discovers the data during execution.

You could use pivot-table techniques, but only if you know how many columns you need for the number of categories per product.

I suggest you fetch the results in rows as in your first example, and then write code in your client application to format it into columns.